The present disclosure relates to an exhaust gas purifier for an engine.
Exhaust gas from lean-burn engines such as diesel engines and lean burn gasoline engines includes NOx (nitrogen oxide). As catalysts for NOx reduction, for example SCR (selective catalytic reduction) catalysts are known in the art. SCR catalysts use NH3 as reducing agent to reduce NOx included in the exhaust gas. In the case of automobiles, aqueous urea solution is widely used as a precursor of NH3. In general, an injector for the aqueous urea solution, a mixer, and an SCR catalyst are provided in an engine exhaust pipe from upstream to downstream in this order. In this configuration, the aqueous urea solution is injected by the injector, mixed with the exhaust gas by the mixer, and supplied to the SCR catalyst. NH3 is generated due to thermal decomposition or hydrolysis of urea.
An example of such an SCR system is disclosed in German Patent Publication No. 102010055642. In this SCR system, an oxidation catalyst, an injector, a mixer, a hydrolysis catalyst, an SCR catalyst, and another oxidation catalyst are provided in an exhaust pipe of an engine from upstream to downstream in this order. In such an SCR system, when the exhaust gas has a low temperature, urea and its derivatives are at risk of precipitating at a tip of the injector which is exposed inside the exhaust pipe. Precipitation of urea and other materials can lead to malfunctions of the injector. In order to prevent the precipitation of urea and other materials, the above publication attempts to reduce the injection amount, injection frequency, and injection pressure of the aqueous urea solution when the exhaust gas has a low temperature.
In order to increase NOx reduction efficiency in an SCR system, the mixer needs to efficiently mix the aqueous urea solution with the exhaust gas. Therefore, in a conceivable configuration, for example, a crank-shaped pipe portion including a first bend and a second bend which are bent in opposite directions is provided in an exhaust pipe, a mixer is arranged in a straight pipe portion continuous with the second bend, and an injector is attached to an outer side wall of the second bend. That is, by configuring the exhaust pipe in a cranked shape, an injector can be arranged such that an injection axis of the injector extends in a longitudinal direction of the straight pipe portion and points toward the mixer. As a result, the mixer can easily mix the aqueous urea solution with the exhaust gas.
However, if the exhaust pipe has a cranked shape, the exhaust gas is prone to take a short route when flowing from an inner side of the first bend along an inner side of the second bend. Thus, the exhaust gas does not flow well in the vicinity of the outer side wall of the second bend the injector is attached to, which is why the flow of the exhaust gas can easily stagnate. Therefore, heat is hardly transmitted from the exhaust gas to a tip of the injector, and, if the exhaust gas has a low temperature, precipitation of urea and other materials is prone to occur at the tip.
In the above publication, by contrast, the injection amount of the aqueous urea solution is reduced as a measure to prevent the precipitation of urea and other materials. This leads to the concern that NOx reduction performance is sacrificed in favor of precipitation prevention. Alternatively, it is conceivable to provide an insulator or to raise the temperature of the tip of the injector by the help of a heater. Such a solution, however, would increase the device in size and also be a disadvantage in terms of costs.